The present invention relates to a nitrogen treating method and system for water to be treated, which contains organic nitrogen, nitrite nitrogen, nitrate nitrogen, nitric acid ion, or ammonia (hereinafter, “water to be treated” will be referred to as “for-treatment water”).
It has been well known that the existence of nitrogen compounds is one of causes of eutrophication of rivers and lakes. The nitrogen compounds much exist in domestic life waste water or industrial waste water, but it is difficult to purify them and there are no effective countermeasures up to date. In general, a biological treatment has been implemented. However, the biological treatment comprises two processes, i.e. a nitrification process for converting ammonia nitrogen to nitrate nitrogen, and a denitrification process for converting nitrate nitrogen to nitrogen gas. Accordingly, there has been a problem that two different reaction vessels are required. There has been a further problem that because a time required for the treatment is extremely long, the treatment efficiency is extremely low.
Further, in this biological treatment, there has been another problem that a large-capacity anaerobic vessel is necessary for keeping denitrifying bacteria, thereby to induce the increase in equipment construction cost and apparatus installation area. There has been a further problem that the denitrifying bacteria are largely influenced by ambient temperature environment, components contained in the for-treatment water, and the like, and in particular, during the winter season when the temperature is low, their activities are lowered to deteriorate the denitrifying action, resulting in unstable processing efficiency.
Accordingly, there has been proposed a method for solving the foregoing technical problems, wherein a current is fed to the for-treatment water to dissolve ammonia, nitrite nitrogen or nitrate nitrogen through oxidation or reduction into nitrogen gas.
However, according to the conventional nitrogen compound treating method based on the electrolysis, there has been a problem that a reverse reaction occurs wherein ammonia is produced at the cathode side while nitric acid ion is produced at the anode side, resulting in lowering of the processing speed. Following this, there has been raised inconvenience due to lowering of the nitrogen removing efficiency.